The invention relates to a brush bag for a current transfer unit of a dynamo-electric machine, comprising a commutator, especially for an electric motor, wherein the brush bag, which is especially made of metal, has guiding areas for guiding a brush which is displaceable with respect to the longitudinal extension, which is especially embodied as a multi-layer carbon brush, and a spacing is provided between the guiding areas and the brush transverse to the longitudinal extension of the brush. In addition, the invention relates to a brush for the brush bag, a current transfer unit for a dynamo-electric machine comprising the brush bag and/or the brush or a dynamo-electric machine comprising such a current transfer unit and a method for producing the brush bag.
The current transfer unit is a so-called sliding contact system which ensures the electrical connection between a power supply or external connections of the dynamo-electric machine and windings of an armature. For this purpose, in known dynamo-electric machines, such as electric motors an arrangement comprising spatially fixed brushes which usually consist of carbon and are in grinding contact with a rotatable commutator are provided. The commutator consists of individual electrically conducting segments which are connected to the windings of the armature. The brush slides along the segments to produce an electrical contact between the segments and the brush.
Different designs of current transfer units are known. One of these designs provides that a brush is slidingly guided in a spatially fixed brush bag which is also designated as a brush guide or brush sleeve. The brush is loaded with a spring in the direction of the commutator, that is in a radial direction. This ensures a substantially uniform contact pressure of the brush on the commutator. Likewise, the brush is tracked by means of the spring force inside the brush bag to compensate for any wear of the brush.
Especially in AC motors, for example, in AC universal motors, so-called multi-layer carbon brushes are used in the known current transfer units. The multi-layer carbon brushes have high-resistance or electrically non-conducting layers between individual carbon layers to reduce so-called shunt currents or short-circuit currents inside the brush. These shunt currents can occur if a brush is in contact with two adjacent segments of the commutator or if a brush or individual brush layers come in contact with an electrically conducting brush bag in such a manner that a current can be formed transverse to the brush. Electrically conducting brush bags, usually made of metal, are frequently used in dynamo electric machines as a result of their inexpensive manufacture. In order to avoid contact between the brush and the brush bag, whereby a shunt current can form in the brush, these brush bags have a relatively large lateral play between the brush and the brush bag. For example, a carbon brush of an AC universal motor of a washing machine is usually 0.04 to 0.05 mm narrower in its transverse extension than the inner guiding area of the carbon brush bag allocated in this transverse extension. In the case of a carbon brush located centrally in the carbon brush bag, the relatively large play between the carbon brush and the carbon brush bag in such a case is therefore 0.02 to 0.025 mm on both sides.
Whilst the brush is used as prescribed, the brush undergoes wear whereby it is abraded. The brush dust thereby produced enters between the brush and the brush bag. In order that the brush cannot jam as a result of the brush dust in the brush bag or so that no shunt currents are induced in the brush by the partially unburned and therefore electrically conducting dust particles, a large lateral play between the brush and brush bag is also favourable. Dust particles which enter into the intermediate space between the brush and the brush bag can easily escape again if there is a large play.
A disadvantage with the large play is that noise is produced during operation of the dynamo electric machine. As soon as the brush impinges upon a segment edge, the brush impacts against the inner walls of the bag. In this case, the brush impacts twice per segment.
A known solution for avoiding noise provides that the commutator surface provided with segments should additionally be treated by so-called pumices or fine grinding. During after-treatment of the commutator, dust particles are again formed which must be laboriously removed so that these dust particles cannot cause a brush jam again.
A brush bag which is especially suitable for guiding multi-layer carbon brushes and which has a spacing transverse to the carbon brush to be guided is known from DE 101 57 604 A1. The brush bag is made of tin-plated steel sheet and is coated with an insulating layer. The insulating layer applied in the brush bag is used to avoid a short circuit current or shunt current between individual layers of a multi-layer carbon brush. A varnish such as phenol resin, epoxy resin or polystyrene varnish is applied as an insulating layer to the steel sheet of the carbon brush bag. When such insulation is used between the carbon brush bag and the multi-layer carbon brush, the intermediate space or the play between the brush bag and the carbon brush can have close tolerances. Such a narrow play avoids any impact of the carbon brush on the brush bag. A disadvantage with the small play is that the brush dust which enters into the intermediate space can deposit in the intermediate space whereby the brush jams in the brush bag or the layers of the brush are short-circuited by unburned brush dust. In addition, such an insulating layer cannot be subjected to such strong thermal and mechanical loading as a pure metal guide.
On the other hand, in the prior art according to U.S. Pat. No. 2,430,279, an arrangement is proposed whereby the brush is laterally pre-stressed towards a brush holder wall by means of a ball-spring element connected to the brush. A disadvantage with this design however is that the spring provided to track the brush inside the brush holder must have a significantly higher spring force than is the case in embodiments which provide a play between the brush and the brush holder. However, the spring force required to overcome the friction between the brush and the brush holder can result in higher wear of the brush on the commutator.